Switching DC-to-DC power converters having a multi-phase coupled inductor topology like that described in U.S. Pat. No. 6,362,986 to Schultz, et al., the disclosure of which is incorporated herein by reference, are known in the art. These converters have advantages, including reduced ripple current in the inductors and switches allowing reduced per-phase inductance or reduced switching frequency, over converters having conventional multi-phase dc-dc converter topologies. Switching DC-to-DC converters as described in U.S. Pat. No. 6,362,986 typically operate in continuous conduction mode (CCM) for high efficiency while driving heavy loads.
Load Variability
DC-DC power converters are often used in applications where the load may vary considerably as a system operates.
For example, the processor of a modern notebook computer may demand tens to more than one hundred amps of current when performing processor-intensive computation at maximum clock rate, while it needs much less current, possibly only a few milliamps, when the system is idle. When a DC-DC converter is designed to power such a processor, the inductors, capacitors, and switching transistors of the converter are typically designed to handle the maximum sustained current required by the processor without overheating.
There are many other applications for power converters where converter load current levels may vary over time. Variation between maximum and minimum load current of factors of hundreds to thousands are not unusual.
Continuous Conduction and Discontinuous Conduction Modes of Operation
Most DC-DC converters that deliver high current operate in Continuous Conduction Mode (CCM). CCM is an operating mode wherein the high and low side switches keep switching on and off alternatively and the current in the output inductor keeps ramping up and down continuously. In CCM in a synchronous DC-DC converter, inductor current never stops flowing, although it may cross through zero. At high current outputs, CCM enables the converter to deliver high current with high efficiency.
In CCM, the inductor carries significant AC current even at low loads. Therefore, there are power losses, such as those due to resistive loss in converter switches and inductor windings, and those due to charging and discharging the parasitic capacitors of the switches, that are present even when the converter operates at low load.
Therefore, with CCM operation, the switching loss and the AC current related loss do not scale down with decreasing load current and they may become a significant part of the total power absorbed by the converter when the load current is small. Since many systems spend considerable portions of their operating lifetime operating at low power levels, they may waste considerable energy over their lifetimes. It is especially important in battery powered systems that DC-DC converters operate at high efficiency over the entire range of possible output power demand to optimize battery life.
Discontinuous conduction mode (DCM) is an operating mode of a DC-DC converter where energy is delivered to the output only when needed. When energy delivery is not required, the switches stop switching and remain off until energy delivery is required. When the switches are off, the current in the inductor remains zero and an output capacitor, the key component of an output filter, supports the output current during the time both switches are off. In this way, switching loss and AC current related loss scale down with decreasing load current and the DC-DC converter maintains high efficiency even at light load.